Power amplifiers in communication systems, such as radio frequency (RF) systems, have limited dynamic ranges; operating at high output power levels leads to phase and amplitude distortion, while operating at reduced output power levels leads to system inefficiency (i.e., the ratio of the output RF power to the combined power from the DC power supply and the input signal is less than optimal). Power amplifiers also produce amplitude and phase distortion at higher input or output power levels. As a result, the efficiency of a power amplifier improves as the dynamic range of the amplified signal is reduced. In general, power amplifiers are most efficient in systems that rely on constant envelope modulation techniques.
Most modern wireless communication standards, including IS-95A, IS-136, and Personal Digital Cellular (PDC), use non-constant envelope modulation techniques. Signals in systems that follow these standards exhibit large dynamic ranges, with peak-to-average power values typically varying from as little as 2.9 dB to as great as 5.8 dB. Since the amplifier must be backed off to faithfully reproduce the peaks, the maximum efficiencies of power amplifiers designed for these systems typically range from 35% to 50%.
One technique for improving the performance (ie., efficiency and fidelity) of a power amplifier in a non-constant envelope environment is the envelope elimination and restoration (EER) technique described by Leonard R. Kahn in "Single-Sideband Transmission by Envelope Elimination and Restoration," Proceedings of the I.R.E., vol. 40, pp. 803-06 (1952). FIG. 1 shows a particular implementation of an envelope elimination and restoration amplifier 100 in which an amplitude limiter 102 and an envelope detector 104 are used to separate a low power RF signal 106 into two components: (1) a phase and frequency modulated (FM) signal 108 having a constant envelope and (2) a baseband signal 110 representing the non-constant envelope. A standard RF power amplifier 112 amplifies the constant envelope FM signal 108, and a envelope amplifier 114 amplifies the envelope signal 110. An envelope combining power amplifier 116 combines the amplified FM and envelope signals. A delay element 118 ensures proper timimg of the signals arriving at the envelope combiner 116. An example of this topology, known as the envelope feedforward amplifier, is described in detail in U.S. patent application Ser. No. 09/108,628, filed on Jul. 1, 1998, by Donald Brian Eidson and Robert Edmund Grange, and titled "Envelope Feedforward Technique with Power Control For Efficient Linear RF Power Amplification."
By breaking the RF signal into a constant envelope RF component and a baseband envelope component, this technique provides a great improvement in amplifier efficiency, yielding actual efficiencies of more than 70% in many systems. Nevertheless, communication systems that produce very large peak-to-average power variations, such as an IS-95B system with eight channels (&gt;17dB variations), greatly reduce the efficiency of power amplifiers that use even this amplification technique. Moreover, many EER techniques cannot support high dynamic range signals and/or still provide additional range for average power adjustment (e.g., average power control in cellular systems).